Electronic amplifier control circuit



y 18, 1943- 1.. HPETRUSCHELL 2,441,840

ELECTRONIC AMPLIFIER conmoflcmcuw Filed Sept. 23, 1944 POWER SUPPLY JAAAAIIA- vvvvvv w L.-

INVENTOR Z Ppezruschell RNEYS I Patented May 18, 1948 STATES PATENT oer ICE.

ELECTRONIC AMPLIFIER GONTROL CIRCUIT,

Lucius P. Petruschell, Mount Dora, Fla... assigncr to, Dictaphone Corporation, New York, N.- Y., a: corporation. of. New York Application September 23, 1944; Serial No. 555563 6.0laims.

This invention relates. to an eleotronicamplifier control system of the ty e commonly referred to. as automatic volume control. andmore particularly'tc such asystem especially adapted to the recording of telephoneconversationsandithe like.

The recording of soundssuch astelepl-ionei conversations presents a particularly difilcult problem because two sounds of. distinctly different magnitudes, a near voice and: a distantvoice; must be amplifiedand recorded by the same: amplifier and" the same recorderz Inasmuch asv the usual recording apparatus requires: aninput signal of substantially constant level toproduce a. satisfactory record, it is necessary to. changethe amplification of' the recording: amplifier each time the voices change so that the input signal to. the recorderwil l be at a substantially constant level regardless of the levelof the: input. signal to the amplifier. This is customarilvdonejby the. provision of an automatic'volume control circuit; which for purposes, of brevity will be referred to hereinafter esan' A. V. C. circuit. In accordance. with usual practice in such. circuits, a portion of the outputs-ignal of the amplifier is'rectifledand fed backthrough aresistance-capacity filter circuit, hereinafter to be refenred to. as. anR.-C.. filter circuit, into the grid circuit of oneofi the amplifier tubes, having a variable-mu characteristic; to chene its grid-bias and thereiorerits; amplification factor in accordancewith the magnitude of? amplifier output signalisoasto maintain that signal at -a'-constant..leel..

In the recording of telephone conversationsthe action. of the A.- VP (3. circuit." changing the plification factor o1"sensitivityozfi"therecording: amplifier as: the near voice: stops speaking and" the distant voice takes upzthe conversation, and vice versa. mustbe fairly rapid in orderthat the first words or syllables are not lost. iorthe' period during which the A. v-0. circuit-is changing the amplifier sensitivity. .is especially true when the distant voice takes the; conversation attorthe nearxvoicexstops because during the. period wh en' the near. voicei's. speaking the amplifier sensitivity is quite low and therefore; probably not sufiicient to provide an adequate signal for the recorderduring the first wordsgor syllables: spoken. by the distant voice; Also, is; desirable that thisperi-od'he short so that the'strong" signal from the near voice will not blast the recorder immedietely after thewdistant voice :stops talking. However. this period mustnot be sozshort, that is the A. V. C. circuit must-notoperete sflzjlfiil'iidly,

that the amplifier regains; tul-l sensitivity during short pauses such as those betweenthe-words 1 a recordable. signal at its output.

If the values-chosen. arelarge, the: A. V. 0.. circuit will respond so slowly that many syllables; if not.

words, of the'distant voice will be; lost, beforethe amplifier gain is: increased sufiici ently to produce If the values chosen are too small, therezisinsufficient filtering the-rectified, signal voltage. from the output of t h'e'ampllfier in. the A. Va. C. circuit causing;.spuri-'- one; modulations of thesignal' due to the rapid. action of the A. V. C. In addition, such small" values cause theoA'. V. voltageimpressed onthegrid; ofthe inputemplifiertube to .drive the grid: potential so farnegative; with-respecttto' its; cathode that it. completely blockspassage of any' si'gnalf when. the near voice replaces the distant voice; and; this excessive negetive'voltage. is maintained fora sufiicient period oftime tointroduce asevere distortion or purse effect inwhich'the recorded signal: starts veryloud and then dies out com.-

pletely" andi finally returns to a normal level. Such short timing of the. A; V. C. R.-C. filter may' even give this. undesirable punch effect before everyword; or after any hesitation in the course of a corrversat-ion. The period. during which an A. V. C. circuit causes the amplifier sensitivity to change from lowga-in to high gain-is frequently call'ed'the releasertnne of an A. V. C. circuit.

Conversely; the period during which an A. V. G. circuit causes the amplifiergain to. be reduced iromaahigh' value to a lowerva'lue is often called the acceptance time of the. A. V. C. circuit; It has beenicund'that if the values'oi' the constants in:theR.-C. filter are chosen soastor'be sufficiently large'tov eliminate the punch effect during'the acceptance period when shifting from a distant or weak signal to aneazror strong. signal, such values" produce a relees'e period which is: too long for signals are not satisfactorily recorded immediafter; the discontinuance of the near or strong signal. For: the most, satisfactory opera;- tion it is; desirable tohave thereleese time suffisi ently short so that weak signals can-be recorded immediately afterthe cessation: of strong signals, and the acceptance time suiiici'cntly long to-avoid the undesirable punch effect. Prior: to the present invention; there has been no Way of" changing the release} time with respect'to: the

acceptance time, an increase in one causing it correspondingincrease in the other, and vice versa;

Accordingly itis an obfr'ectof the present inventionto provide an A. V. C. electronic amplifier cont-r01: system in. which the-release time" of the" A. V. C. circuit may be-va-ried with respectito the acceptance time: of the circuit;

This and other apparent objects and adv n tages of thisrinventionare obtained by the means described; the following specification may be more; readily understood by:- refienenc to the? atisfa ctory operationzso that distant-or weak:

accompanying drawing which shows schematically the wiring diagram of an electric telephone conversation recording apparatus embodying the present invention.

Referring to the drawing, 2 is a standard tele- A phone instrument of the ringer-in-base type inductively coupled to a recording amplifier, generally indicated at 4, by means of an inductive telephone pick-up 6 such, for example, as that described in U. S. Patent No. 1,782,619 to M. J. Johnson. The recording amplifier'4 is coupled by any suitable means such as an output transformer 8 to a suitable electrical recorder such, for

example, as that shown and described in U. S. Patent No. 1,909,839 to J. Motyl. Operating potentials are provided for amplifier 4 from a suitable power supply, generally indicated at I2. An A. V. C. circuit, generally indicated at 14, is connected from the output of amplifier 4 to the first tube l6 of the recording amplifier 4 as will now be described in greater detail.

' A voltage proportional to the output of amplifier 4 is fed back through leads l8 and to the A. V. C. circuit 14 from some suitable point in the amplifier output such, for example, as across the secondary of output transformer 8. Lead I 8 is connected to the ground circuit of amplifier 4,

and lead'2ll is connected through a current limit-' ing resistor 2|, a rectifier 22 and a grid-biasing resistor '24 to the ground circuit. Bias resistor 24 is shunted by afilter condenser 26. Thus a D. C. voltage is produced across resistor 24 which is proportional to the output voltage of amplifier 4. This voltage is applied as a negative grid bias to the control grid 28 of the first tube iii of amplifier 4 through an R.-C. filter circuit, comprising a resistor 30 and a condenser 32, and the grid resistor 34 of the tube l6, as shown in the drawing. The circuit thus far described comprises an A. V. C. circuit frequently used in electronic amplifiers. The rapidity of its response is primarily determined'by the values of resistor 30 and condenser 32. Condenser 32 must be charged through resistor 30 before the full D. 0. potential developed across grid-bias resistor 24 is applied 7 to the grid 28 of vacuum tube 16 to reduce its amplification factor by an amount proportional to the voltage developed across resistor 24 and therefore to the voltage produced at the output of amplifier 4. The choice of the constants in this filter circuit, formed of resistor 33 and condenser 32, will affect the release time and the acceptance time of the A. V. C. circuit in the manner above described. In order to produce an acceptance time of sufficient duration to avoid the undesirable punch effect, constants of considerable size must be used. For example, a value of 2.5 megohms for resistor 30 and a value of l microfarad for condenser 32 have been found to be s'ufliciently large to produce an acceptance time which eliminates the punch effect. When such values are used, however, the release time is of such magnitude that a considerable portion of a weak signal, when it follows immediatley. upon the cessation of a strong signal, is lost before the A. V. C. circuit has increased the gain of the amplifier 4 sufficiently to produce a recordable signal at the output of the amplifier. In accordance with the present invention in order to vary the release time with respect to the acceptance time, a second rectifier'36 is connected in series with a resistor 38 acrossre'sistor 30 as shown in the drawing. In the present embodiment of the invention in order to produce a release time shorter than the acceptance time, resistor 38 has a value considerably less than the value of resistor 30. For example, with an R.-C. filter circuit having the values above-mentioned, it has been found desirable to make resistor 38 equal to about 0.2 megohm to give a suitably short release time, that r is about one fourth of a second as contrasted with an acceptance time of more than two seconds.

' The operation of this circuit is readily apparent. When no signal is applied to amplifier 4, and therefore no voltage is produced at its output, no voltage appears across bias resistor 24. Thus a minimum negative bias is applied to grid 28 of tube l3 so that it is in condition for maximum amplification. When a strong signal is applied to amplifier 4, a voltage will appear at its output and a D. C. voltage will appear across bias resistor 24. As soon as condenser 32 is charged through resistor 30, this negative D. C. voltage is applied to the grid 28 of tube 16 so that its amplification factor is suitably reduced. The time in which this reduction is accomplished, that is, the acceptance time of the A. V. C. circuit I4, is determined by the time constant of the R.-C. filter circuit formed as resistor 30 and condenser 32. In the example given above the time constant of this circuit is about 2.5 seconds. During this operation, resistor 38 is not effective because rectifier 36 is non-conductive for its anode is negative with respect'to its cathode. However, when the strong signal applied to amplifier 4 is superseded by a weak signal, the D. C. voltage across bias resistor 24 drops to a lower value and condenser 32 tends to discharge through resistor 30 thus reducing the negative bias on the grid 28 of tube I3 and thereby increasing its amplification factor. Without the provision of rectifier 3-6 and resistor 38, this reduction in negative bias on the grid 28 of tube i6 would be accomplished in a'tinie equal to the acceptance time of the A. V. C. circuit I4. However, by means of the rectifier 36 and resistor 38 this release time of the A. V. C. circuit i4 is greatly reduced because condenser 32 is able to discharge very rapidly through the relatively low resistance 38 and rectifier 36 because, under the conditions just described, the anode of rectifier 36 is positive with respect to its cathode. With this arrangement the exact release time is determined by the time constant of the R.-C. filter circuit formed by condenser 32 and resistors 33 and 38 in parallel. .In the illustration given above, thisrelease time constant is approximately one fourth of a second as compared to the acceptance time constant of 2.5 seconds.

Although rectifiers 22 and 33 have been shown as vacuum diodes, any suitable rectifiers might be used. In addition, although these vacuum diodes have been shown as separate tubes, it is evident that a single tube containing two diode units could be substituted for the two tubes shown.

Thus, in accordance with the present invention it can be seen that the release time of an A. V. C. circuit can be set at a different value from its acceptance time. In the embodiment shown the acceptance time is determined by the value of resistor 30 and condenser32 whereas the release time is determined by these values and the value of resistor 33. Should it prove desirable to make the acceptance time of the A. V. C.

shorter than its release time, in accordance with the present invention, this could be accomplished by reversing the rectifier 36.

As many embodiments may be made in the above invention and asmany changes may be made inthe embodiment above described, it is to be understood that all matters hereinbefore set forth or shown in the accompanying drawing are to be interpreted as illustrative only and not in a limiting sense.

I claim:

1. In a sound recording system, the combination of an electronic tube amplifier, an electric recorder connected to the output of said amplifier, an A. V. C. circuit including a feedback connection from one side of the output of said amplifier to ground and from the other side of the output of said amplifier to the cathode of first diode rectifier, a first resistor connecting the anode of said first diode to ground, a second resistor connected from the anode of said first diode to the grid return of at least one of the tubes ofsaid amplifier, a condenser connecting the grid return side of said second resistor to ground, a second diode rectifier in series with a third resistor connected as a shunt across said second resistor with the anode of said second diode connected to the anode of. said first diode and the cathode of said second diode connected to the grid return side of said second resistor whereby the release time of said A. V. C. circuit is caused to be shorter than its acceptance time.

2. In a sound recording system, the combination of an electronic tube amplifier, an electric recorder connected to the output of said amplifier, an A. V. C. circuit comprising a feedback connection from the output of said amplifier to a rectifier and a bias resistor connected in the grid circuit of at least one of the tubes of said amplifier for supplying a negative bias to said grid circuit in proportion to the magnitude of the output signal of said amplifier, first means for determining the acceptance time of said A. V. C. circuit, and second means for determining the release time of said A. V. C. circuit.

3. In a sound recording system, the combination of an electronic tube amplifier, an electric recorder connected to the output of said amplifier, an A. V. C. circuit comprising a feedback connection from the output of said amplifier to first rectifier means and biasing means in the grid circuit of at least one of the tubes of said amplifier for supplying a negative bias to said grid circuit in proportion to the magnitude of the output signal of said amplifier, first means for determining the acceptance time of said A. V. C. circuit including an R.-C. filter circuit comprising a resistance and a capacity connecting said bias resistor to said grid circuit, and second means for determining the release time of said A. V. C. circuit including second rectifier means in series with resistance means shunting said resistance portion of said R.-C. filter circuit to alter the discharge time of the capacity portion of said R.-C. filter circuit with respect to its charging time, whereby the acceptance time of said A. V. C. circuit is altered with respect to its release time.

4. In combination with an electronic tube amplifier, an A. V. C. circuit including a connection supplying a signal voltage from said amplifier through a first rectifier to a control resistor across which an A. V. C. voltage is developed, an R.-C. time delay filter circuit formed of a condenser and a first time delay resistor, said condenser being connected in the grid circuit of at least one of the tubes of said amplifier and being adapted to be charged to the potential of said A. V. C. voltage through said first time delay resistor, a

second time delay resistor in series with a sec-.

ond rectifier connected in parallel with said first resistor, whereby current flowing to and from said condenser flows through said first time delay resistor only when flowing in one direction and through said second rectifier and second time delay resistor in parallel with said first time delay resistor when flowing in the opposite direction to alter the acceptance time of said A. V. C. circuit with respect to its release time.

5. In a telephone conversation recording apparatus, the combination of an electronic tube amplifier, means for coupling the input of said amplifier to a telephone circuit, an electric recorder connected to the output of said amplifier, an A. V. C. circuit for supplying a negative bias to the grid of at least one of the tubes of said amplifier, comprising a feedback connection from the output of said amplifier through a first rectifier and a biasing resistor connected in the grid circuit of said tube, an R.-C. filter circuit comprising a first high resistance interposed in said grid circuit in series with said biasing resistor, and a condenser shunting said biasing resistor and said series resistor, a second rectifier in series with a second high resistance shunting said first high resistance whereby current flowing to and from said condenser flows through said first high resistance when flowing in onedirection and through said first and second high resistances in parallel when flowing in the reverse direction to cause the release time of said A. V. C. circuit to differ from its acceptance time.

6. In a telephone conversation recording apparatus, the combination of an inductive telephone pick-up unit, an electronic tube amplifier having its input terminals connected to said pick-up unit, an electric recorder connected to the output of said amplifier, an A. V. C. circuit for supplying a negative bias to the grid of one of the tubes of said amplifier, comprising a feedback connection from the output of said amplifier through a first rectifier and a, biasing resistor connected in the grid circuit of said tube, an R.-C. filter circuit comprising a first high resistance interposed in said grid circuit in series with said biasing resistor, and a condenser shunting said biasing resistor and said series resistor, a second rectifier in series with a second high resistance shunting said first high resistance, the anodes of said first and second rectifiers being connected together, and to said biasing resistor, whereby said condenser is charged through said first high resistance and discharged through said first and second high resistances in parallel to cause the release time of said A. V. C. circuit to be shorter than its acceptance time.

LUCIUS P. PETRUSCHELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,137,401 Hobbie Nov. 22, 1938 2,219,729 Tahon Oct. 29, 1940 2,244,695 Hathaway June 10, 1941 2,250,559 Weber July 29, 1941 2,278,828 Chiarclio Apr. 7, 1942 2,319,665 Dobrzensky May 18, 1943 2,332,536 Schlegel Oct. 26, 1943 2,340,159 Thompson Jan. 25, 1944 

